Enhanced heavy metals sorption by modified biochars derived from pig manure

To enhance the sorption efficiency of heavy metals by modified biochar, biochars derived from pig manure at different pyrolyzing temperatures (300 °C, 500 °C and 700 °C) were oxidized and thiolated using H2O2 and 3-mercaptopropyltrimethoxysilane, respectively. The physicochemical properties of biochars and sorption kinetics, isotherms, and mechanisms for Pb and Cd were studied. Oxidation and thiolation decreased the aromaticity and ash content, and increased the polarity and the specific surface area and pore volume of biochars. Oxidation introduced oxygen-containing functional groups into biochars, and the oxidation reaction took place at CC bonds. The thiolation of oxidized biochar prepared at moderate-temperature had the highest sulfur content (4.43%) due to that this biochar was rich in oxygen-containing groups, which can act as the active sites for thiol groups loading. Sorption capacity of biochars for heavy metals increased by 2.58–4.22 times with the increase of pyrolyzing temperature, except the original biochar prepared at moderate-temperature, which had the highest Pb sorption capacity. The distribution coefficient of heavy metals onto modified biochars was enhanced by up to 85.4%. Oxidization treatment was more suitable for immobilization of low concentrations of Cd onto low- and moderate-temperatures biochars, while thiolation treatment was more suitable for immobilization of high concentrations of heavy metals. The sorption of heavy metals by biochars occurred via cation exchange, precipitation, cation-π EDA interactions and complexation. The contribution percentage of complexing with functional groups increased after modification. The results suggest that oxidization and thiolation treatments of animal manure biochars are promising methods for the treatment of heavy metals polluted environment.